AppleTV+

How many smart TV's can you connect up to an Apple TV+ account [?}

As many devices as you have. See the list of options here.

However Apple states here that "One subscription allows you to enjoy 6 simultaneous streams." Six is also the number you can have in a Family Sharing group, so all members of the group can use Apple TV+ steaming at the same moment on different devices. I guess it happens.